updatede readme

packages/gatsby-image/README.md

@@ -259,6 +259,7 @@ prop. e.g. `<Img fluid={fluid} />`
 | `position` | `string` | Defaults to `relative`. Pass in `absolute` to make the component `absolute` positioned |
 | `backgroundColor` | `string` / `bool` | Set a colored background placeholder. If true, uses "lightgray" for the color. You can also pass in any valid color string. |
 | `onLoad` | `func` | A callback that is called when the full-size image has loaded. |
+| `onStartLoad` | `func` | A callback that is called when the full-size image starts loading, it gets the parameter { wasCached: boolean } provided. |
 | `Tag` | `string` | Which HTML tag to use for wrapping elements. Defaults to `div`. |
 
 ## Image processing arguments